Precision abrasive finishing apparatus

ABSTRACT

A precision abrasive finishing apparatus comprising a base, preferably of granite and being ground to true flat; an abrasive lap capable of being vacuum-clamped against said base and having an abrasive material disposed on it upper, lapping surface; and vacuum clamping means. The abrasive material may be any conventional abrasive grit, such as aluminum oxide, silicon carbide, cubic boron nitride, or other material of suitable hardness, sharpness, friability, durability, inertness, and cost, but preferably is diamond.

FIELD OF THE INVENTION

The present invention relates to a precision abrasive finishing apparatus, specifically, to a lap surface capable of being retrofitted to a conventional lapping table or to a curved mold.

BACKGROUND OF THE INVENTION

Lapping is a time-honored way of refining the geometry and finish of work pieces. Typically, the work piece is rubbed over a flat abrasive base surface that is larger than the work piece. The flat base surface may be made of an abrasive material that is either quarried or man-made, or the flat base may not itself be abrasive, but rather be coated with an abrasive material. Randomized motion is provided either by hand over a stationary abrasive base or by machine movement of either or both of work piece and base. The base should be flat-surfaced and a base having any deviation from true flat will degrade the resulting flatness and finish of the work piece.

The abrasive material may be grain-locked in a rigid three-dimensional matrix or grains may be carried in water, oil, or other fluid as a slurry or bonded to a thin carrier substrate by gluing, brazing, or codepositing in metal electroplate or by embedding the abrasive grains into a softer metal surface by action of a hard surfaced roller. The abrasive grains may be aluminum oxide, silicon carbide, diamond, cubic boron nitride, or other material of suitable hardness, sharpness, friability, durability, inertness, and cost.

Typically, lapping involves a series of finishing passes using progressively smaller sized abrasive grains so that undesirable work piece material is removed by a multitude of cuts, producing chips (“swarf”) and leaving scratches that become progressively finer until the desired flatness and finish are achieved. This stepping down in grain size is achieved either by changing the slurry, changing the abrasive coated thin substrate, or moving the workpiece to different laps. Another option is to use a slurry having a friable grain that mechanically breaks down during the lapping process, effectively providing progressively smaller grains and progressively finer scratches.

One major drawback to using abrasive slurries is that they abrade the flat base surface as well as the work piece and periodic re-truing of the base is necessary. Abrasive slurries also require periodic replacement due to undesired grain breakdown and to loading with swarf. Bonded abrasive papers and films held to base flat surfaces by mechanical tension, water “stiction”, or adhesives also require replacement due to grain breakdown and displacement. Although bonded abrasives do allow flushing with water or other fluid to remove swarf, they are still subject to the problem of the grains themselves being abraded from the carrier medium. The working surface of abrasive paper or film is typically near parallel to the underlying flat base surface with some deviation due to curl, water thickness variation between the paper or film and the flat base surface, or variation in thickness of the attaching adhesive, as the case may be. Each of these factors may result in a working surface that is not true-flat.

The flat base surface may be steel plate, cast iron, float glass per the Pilkington process, aluminum jig plate, composites of materials, or granite. The choice of base material is influenced by its stability under load variation, during changes in temperature, and over time. Hardness, stiffness, cost, and “machinability” are also significant factors that contribute to the choice of base surface material. Laps made of metal or certain other materials that are made abrasive by embedding the surface with abrasive such as diamond are referred to as “charged laps.” Charged laps may prove fairly durable, especially when charged with diamond, because the diamond is generally restrained from abrading the lap once the diamond is securely lodged within the interstices of the lap surface. However, such laps do require periodic recharging.

Granite is the universal standard of flat reference materials in metrology, but is limited in lapping applications to the support of abrasive papers and films because any direct rubbing would wear the granite out of true flat at an unacceptable rate. However, abrasive papers and films have a shorter useful life than other lapping abrasives and require more intensive labor to achieve the same lapping effect.

What is needed is a means for using a true flat granite base for lapping applications that does not employ abrasive papers and films and that does not abrade the granite base. The ideal lapping apparatus would employ a durable abrasive substrate overlaying a true-flat granite base, such that the abrasive substrate is held securely and reliably without slippage during the lapping process. Finally, the ideal lapping apparatus would be capable of easy and quick exchange of abrasive substrates, as where progressively finer grain size is desired or where the useful life of the abrasive substrate has been reached.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a means for using a true flat base for lapping applications that does not employ abrasive papers and films.

It is an additional object of the present invention to provide a means for using a true flat base for lapping applications that does not abrade the granite base.

It is a further object of the present invention to provide a lapping apparatus that allows use of super-abrasives such as diamond.

It is another object of the present invention to provide a lapping apparatus that employs a durable abrasive substrate overlaying a true-flat base.

It is yet an additional object of the present invention to provide a lapping apparatus that allows swarf flushing without significant loss of abrasive material.

It is a still further object of the present invention to provide a lapping apparatus that holds the abrasive substrate securely and reliably without slippage during the lapping process.

It is yet another object of the present invention to provide a lapping apparatus capable of easy and quick exchange of abrasive substrates.

The present invention takes advantage of stable materials such as granite by protecting the flat base surface from wear, allows use of super-abrasives such as diamond, provides quick change of abrasive grain, allows swarf flushing, and delivers these benefits with economy. The abrasive material, such as diamond, is bonded to one side of a precision metal shim and the reverse side is securely held against the flat base surface using vacuum clamping technology. An air evaluation channel is provided within and near the perimeter of the shim's area by milling a trough or channel into either the flat base surface or the opposing backside of the shim.

In the case of a new flat base (granite or metal) the air evacuation channel should be manufactured into the flat base so that any abrasive coated shim may be attached by vacuum chucking. In the case of retrofit of diamond shim to existing flat base, it may be more practical to manufacture the air evacuation channel into the back side of the abrasive coated shim or to manufacture a uniformly thick spacer having air evacuation channels on each face and positioning such spacer between the diamond abrasive shim and the flat base.

Any arrangement of air evacuation channel or channels must be provided with porting to a flexible tube that connects to a vacuum pump or equivalent.

Currently, applications of vacuum chucking or work-holding employ a grid of air evacuation channels. Such a grid of channels was found to be unnecessary in the present invention. Only one air evacuation channel loop per suction face is generally needed. Although a grid of channels may be used on the flat base surface, provided that any grid extending beyond the shim area is blocked off by plugging with temporary filler, it is not necessary to completely underlay the shim with vacuum channels in order to achieve the desired immobilization of the shim. An air evacuation channel loop is needed in each diamond shim if already available granite surface plates are to be retrofitted in the field without modification to the granite. Channel-equipped shim will typically have a corner unavailable for lapping as the channel will need port provision at a corner and emerging from the abrasive side of the shim with suitable fitting and connecting tube routed on to the vacuum source. The abrasive shim may be made from common shim of steel or brass by electroplating a coat of nickel with diamond co-deposits. Alternatively a shim of aluminum may be diamond coated per U.S. Pat. No. 3,287,862 to Abernathy. The diamond may have continuous cut or interrupted cut pattern per the teachings of U.S. Pat. Nos. 4,047,902 to Wiand or 3,860,400 to Prowse. Certain woven textiles of metal or non-metal thread with non-metal in-fill, of near uniform thickness, and interrupted cut of diamond electroplated to the metal thread or nickel islands mask plated so as to entrap non-metal thread by positioning a cathodic surface on the cloth back side during plating are available on the market and suitable for use in a system of the present invention in lieu of diamond shim.

Alternatively, the abrasive surface may be created by spreading diamond compound over a soft shim of copper or of soft steel and embedding the diamond into the shim with a hard steel roll, either before or after vacuum chucking to the flat base. The diamond in this charge may be in the 10-micron to fractional micron range where conventional electroplate coating becomes impractical. When this micron range of diamond is charged into the soft metal shim and held precision flat by the above vacuum chucking to a flat base, the result is a polishing class of lap.

A ceramic-coated shim per U.S. Pat. No. 5,616,229 to Samsonov et al. could be employed in accordance with the present invention, especially as an alternative to charging micron diamond into a shim. The vacuum holding of a shim coated with diamond or otherwise coated with a hard surface may be employed as a wear-resistant surface that can easily be replaced in the event of excessive wear. Thus a metrology surface plate may consist of a granite plate and vacuum-held shim with diamond wear coat or aluminum shim with an anodized wear-resistant surface or Samsonov-surfaced shim or other hard-faced shim.

Either magnetic materials or application of at least a partial vacuum may be employed to force ferrous shim or ground stock against a precision lapping surface. However, magnetic chucks are costly and magnetic forces transmitted through to the abrasive side of the shim or ground stock would impede swarf clearance. More desirably, vacuum techniques are generally lower in cost and work irrespective of the material composition of the shim or ground stock used.

Numerous other objects, features, and advantages of the present invention will become readily apparent from the following detailed description of the invention taken in conjunction with the claims, and from the accompanying drawings in which like numerals are employed to designate like parts throughout the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A shows a plan view of a first embodiment of the present invention wherein a diamond-coated shim is held by means of vacuum chucking over a flat base having air evacuation means cut into the base.

FIG. 1B shows an front elevation view of the embodiment of FIG. 1A.

FIG. 1C shows a cross-sectional view of the cross-sectional view of of the embodiment of FIG. 1A taken through the line A-A′.

FIG. 1D shows an enlarged view of the cross-sectional view of FIG. 1C.

FIG. 2A shows a plan view of a second embodiment of the present invention wherein the air evacuation means necessary for vacuum chucking is disposed within the back side of the abrasive coated shim.

FIG. 2B shows an front elevation view of the embodiment of FIG. 2A.

FIG. 2C shows a cross-sectional elevation view of the embodiment of FIG. 2A taken through the line A-A′.

FIG. 2D shows an enlarged view of the cross-sectional elevation view of FIG. 2C.

FIG. 3A shows a plan view of a variation of the second embodiment shown in FIGS. 2A-D, such that a cap is in fluid communication with both the air evacuation channel disposed within the back side of the abrasive coated shim and the vacuum source.

FIG. 3B shows a cross-sectional view of the embodiment of FIG. 3A taken through the line A-A′.

FIG. 3C shows a cross-sectional view of the embodiment of FIG. 3A taken through the line B-B′.

FIG. 4A shows a plan view of a third embodiment of the present invention wherein a uniformly thick spacer is employed having air evacuation channels on each face and positioned between the diamond abrasive shim and the flat base.

FIG. 4B shows a cross-sectional elevation view of the embodiment of FIG. 4A taken through the line A-A′.

FIG. 4C shows a cross-sectional plan view of the view of FIG. 4C taken through the line B-B′.

FIG. 4D shows an enlarged view of the cross-sectional view of FIG. 4B.

FIG. 4E shows a cross-sectional view of a variation of the embodiment of FIG. 4A, also taken through the line B-B′.

FIG. 5A shows a plan view of a fourth embodiment of the present invention wherein a plurality of shims are capable of being held by means of vacuum chucking over a flat base having air evacuation means cut into the base.

FIG. 5B shows a cross-sectional view of one variation of the embodiment of FIG. 5A taken through the line A-A′.

FIG. 5C shows a cross-sectional view of a second variation of the embodiment of FIG. 5A taken through the line B-B′.

DETAILED DESCRIPTION OF EMBODIMENTS

While the precision abrasive finishing apparatus of the present invention is susceptible of embodiment in many different forms, there is shown in the drawings and will be described herein in detail, a preferred embodiment of the invention. It should be understood however, that the present detail, a preferred embodiment of the invention. It should be understood however, that the present disclosure is to be considered an exemplification of the principles of the invention and is not intended to limit the spirit and scope of the invention and/or claims of the embodiments illustrated.

In its simplest form, as shown in FIGS. 1A-1D, the present invention comprises a base 10, preferably of granite and being ground to true flat, having an upper surface capable of engaging abrasive lap 20. Abrasive lap 20 has a lower surface that substantially lies along said upper surface of base 10 and an upper surface having disposed thereon abrasive material 30. Abrasive material 30 may be any conventional abrasive grit, such as aluminum oxide, silicon carbide, cubic boron nitride, or other material of suitable hardness, sharpness, friability, durability, inertness, and cost, but preferably is diamond.

Base 10 has disposed along its upper surface channel 40 formed as a groove in the upper surface of base 10, such that channel 40 completely underlies the area occupied by abrasive lap 20. Base 10 is further provided with at least one port 50 in fluid communication with channel 40 and the exterior of base 10 that is not overlaid by abrasive lap 20. In this manner, a vacuum may be applied through port 50 to channel 40 by means of hose 60. Additionally, channel 40 may comprise any number of passages in any effective arrangement, provided that each passage is in fluid communication with port 50.

Once abrasive lap 20 is disposed upon the upper surface of base 10, channel 40 may become effectively airtight such that application of a vacuum to channel 40 will result in the suction of abrasive lap 20 to base 10 whereby abrasive lap 20 is essentially clamped to base 10. This process may be facilitated and enhanced by the disposition of a fluid, such as water or light oil, to either the upper surface of base 10, the lower surface of abrasive lap 20, or both.

This embodiment is ideally suited to newly manufactured lapping tables. An air evacuation channel and port may be formed into a flat base, ideally made of granite manufactured to true flat. The port would be capable of connection to the vacuum source via plastic tubing or other fluid connection means. This new system would allow easy exchange of abrasive shims for lapping purposes or used as a measurement surface without an abrasive shim.

EXAMPLE 1

A 6″×18″×0.012″ thick sheet of steel shim was vacuum clamped to ground-flat aluminum jig-plate of similar area by milling a 0.006″ deep×0.020″ wide channel loop into the jig-plate just within the footprint of the shim's perimeter. This channel loop was in fluid communication with the exterior of the jig-plate by means of an intersection bore into the inch thick jig-plate. A plastic tube was fitted to the portion of the bore that exited from the jig-plate and then connected to a vacuum pump. Clamping was quickly achieved by activating the pump and manually pressing down on the shim to close off gaps between the shim and jig plate sufficiently until evacuation and equilibrium partial vacuum resulted. The holding power of the vacuum clamp was sufficient for the steel shim to resist lapping forces horizontal to the surface and remain in place over the channel loop.

An second embodiment, shown in FIGS. 2A-2D, may be used to retrofit existing lapping surface plates to permit the occasional use of a lapping shim 120, as where the primary function of the plate is for measuring (metrology). Shim 120 has an upper surface on which is provided abrasive material 130 and a lower surface having incorporated therein both air evacuation channel 140 and port 150 along the periphery of the shim and exiting upwards, such that it is in fluid communication with a vacuum source via flexible tube 160. Optimally, fitting 155 will be partly disposed within port 150 such that it forms a “nib” to which hose 160 may be fitted. In an alternate to this second embodiment, shown in FIGS. 3A-3C, housing 270 having port access 290 and chamber 280, is disposed above port 250 such that port access 290 is in fluid communication with channel 240. Hose 260 may either directly engage port access 290 or port access 290 may be provided with fitting 295 (not shown) to form an exterior nib to which hose 260 may be fitted. This alternative provides a more robust means by which hose 260 may be fluidly connected to shim 220. Ideally, housing 270 is capable of withstanding occasional impact by the work piece being lapped. However, because housing 270 is biased against shim 220 by means of suction provided to both housing 270 and shim 220 through hose 260 to chamber 270, it is anticipated that should a lateral impact force sufficient to overcome the suction force be imparted by the work piece to housing 270, neither housing 270, shim 220, nor base 210 would be damaged. The user could merely re-establish the vacuum connection by proper arrangement of the various elements and resume the lapping process. This feature also enables quick replacement of shim 220 when either the abrasive surface ceases to be fully functional and shim 220 needs to be replaced, or when a shim having a different grain size is desired.

An additional benefit of this embodiment is that shim 220 may be thinner in cross-section than if port access 290 were disposed through the lateral side of shim 220. Thus, abrasive shim made from precision-rolled shim or slightly thicker precision-ground stock may be used.

A third embodiment, shown in FIGS. 4A-4D enables a retrofit of existing surface plate lapping tables to enable frequent use of abrasive shims and ready exchange of shims having different grain sizes. In this embodiment, abrasive shim 420 does not incorporate an air evacuation channel. Instead, spacer 440 having upper, lower, inner, and outer surfaces is interposed between abrasive shim 420 and surface plate 410. Spacer 440 is provided with air evacuation channels 441 and 442, respectively, formed by milling, grinding, laser ablation and their equivalents within each of the upper and lower surfaces. Channels 441 and 442 are in fluid communication by means of at least one transverse port 443. Spacer 440 may be manufactured from commonly available ground stock (steel) of sufficient thickness to accommodate back-to-back air evacuation channels 441 and 442, leaving some structural wall between these channels, and also to accommodate horizontal port 450 for fluid connection to the vacuum source via flex tube 460. Tube 460 is optimally disposed parallel to flat base 410 and is not thicker than spacer 440, to ensure that tube 460 is not contacted by the work piece during lapping. This system allows easy and frequent change of shims or easy return to employing the flat base as a conventional surface plate in metrology.

Optionally, as shown in FIG. 4E, pliable sealing means 455 may be disposed against the outer lateral surface of spacer 440 at least about the region of spacer 440 adjacent horizontal port 450. Pliable sealing means 445 may be any suitable material, such as various putties, tubular “weatherstripping” materials, and the like. Pliable sealing means 455 may be used to enhance the vacuum seal of spacer 440 solely in the vicinity of port 450 or about the entire outer perimeter of spacer 440, to maximize the suction force experienced by shim 420 and base 410.

EXAMPLE 2

A 6″×18″×0.012″ thick sheet of steel shim has been vacuum attached to an existing granite surface plate of larger area using 5⅝″×17⅝″×0.125″ thick spacer of precision-ground stock having a 0.125″ wide×0.035″ deep channel section cut 0.070″ in from the perimeter of each side of the granite surface plate. A perimeter seal of common (sponge) elastomeric weather strip material was placed about the perimeter. The opposing channels were ported together and to a plastic tube that connected to the vacuum pump.

A fourth embodiment, shown in FIGS. 5A-5C, contemplates use of multiple abrasive shims 520-520 ^(n). The purpose of using multiple shims is to extend the size of the lapping surface beyond the size restrictions of conventionally available shim stock and ground stock. Thus, apparatus of the present invention used in conjunction with conventional lapping tables may enable the user to employ nearly, the entire surface, if not all, of the lapping table, as with conventional lapping techniques, such as use of slurries. There are a variety of means by which multiple abrasive shims 520-520 ^(n) could be vacuum clamped to base surface 510. First, a plurality of grooves and ports may be formed in the working surface of base 510. The plurality of ports 530 may be disposed either laterally along the side of base 510, as shown in FIG. 5B, or through the thickness of base 510, as shown in FIG. 5C, or any desired combination of the above. In addition, a plurality of abrasive shims constructed in accordance with the embodiments of FIGS. 2, 3, and 4 may be used, provided, however, that the required coplanarity of the multiple shims is achieved. If a spacer in accordance with the embodiment shown in FIG. 4D is used, it is desired that the spacer have a “windowpane” construction such that shims 520-520 ^(n) may be disposed thereon such that effective vacuum clamping is achieved. It is understood for purposes of this embodiment that although shims 520-520 ^(n) are pictured as being rectangular, it is contemplated that they may be of circular or any suitable polygonal geometry, such as triangular, square, tetragonal, etc., provided that they are disposed in the desired coplanar arrangement. Additionally, shims 520-520 ^(n) may be arranged such that they provide either continuous or interrupted cut.

Any of the above embodiments can feature a variety of patterns in the abrasive surface for swarf-clearing and fast “interrupted cut” or the non-patterned “continuous cut” for smoother ride of smaller parts in lapping. The shims in the above examples could have been coated with abrasive material per commercial practices or they could have been charged with abrasive compound, as described above.

It will now be apparent to those skilled in the art that other embodiments, improvements, details and uses can be made consistent with the letter and spirit of the foregoing disclosure and within the scope of this patent, which is limited only by the following claims, construed in accordance with the patent law, including the doctrine of equivalents. 

1. A precision abrasive lap for use with a conventional lapping table base having an upper and a lower surface, the invention comprising a precision shim having a perimeter, an upper surface to which an abrasive material is bonded, and a lower surface that enables said shim to be securely held against said upper base surface using vacuum clamping technology.
 2. The precision abrasive lap, as claimed in claim 1, wherein said lower shim surface is provided with air evaluation means within and near the perimeter of the shim
 3. The precision abrasive lap, as claimed in claim 2, wherein said air evacuation means is a channel that is milled into said lower shim surface.
 4. The precision abrasive lap, as claimed in claim 1, wherein said upper base surface is provided with air evaluation means such that said shim may be securely clamped to said upper base surface by means of vacuum clamping.
 5. The precision abrasive lap, as claimed in claim 1, further comprising a spacer having an upper surface and a lower surface disposed between said shim and said upper base surface and capable of securely holding said shim against said upper base surface by means of vacuum clamping.
 6. The precision abrasive lap, as claimed in claim 5, wherein air evaluation means are disposed within each of said upper and lower surfaces of said spacer.
 7. The precision abrasive lap, as claimed in claim 5, wherein said spacer is composed of materials capable of forming a vacuum seal selected from the group consisting of, elastomeric, rubber, polymeric, gel, metal, and ceramic.
 8. The precision abrasive lap, as claimed in claim 1, wherein said lower shim surface is provided with a plurality of protrusions that bias said lower shim surface away from said base surface such that air evacuation means are defined therebetween.
 9. The precision abrasive lap, as claimed in claim 1, wherein said abrasive material is diamond.
 10. A precision abrasive lap for use with conventional lapping table bases having a lapping surface, the invention comprising a plurality of precision shims, each having a perimeter, an upper surface to which an abrasive material is bonded and a lower surface that is capable of being securely held against the upper base surface using vacuum clamping technology.
 11. The precision abrasive lap, as claimed in claim 8, wherein said plurality of precision shims have a geometry that is conducive to coplanar arrangement.
 12. The precision abrasive lap, as claimed in claim 9, wherein said geometry is circular.
 13. The precision abrasive lap, as claimed in claim 9, wherein said geometry is polygonal. 